In the technology of pressure measurement, a distinction is made between difference, absolute and relative pressure measuring transmitters. Pressure difference measuring transmitters serve for measuring the difference between two different pressures. In the case of absolute pressure measuring transmitters, a pressure to be measured is registered absolutely, i.e. as a pressure difference relative to a vacuum. With a relative pressure measuring transmitter, a pressure to be measured is sensed in the form of a pressure difference relative to a reference pressure. The reference pressure is, as a rule, an ambient pressure existing at the location where the relative pressure measuring transmitter is located. In the case of most applications, this is the atmospheric pressure at the site of use. If the sensor is, however, installed e.g. in a space under positive or negative pressure, then the reference pressure is the internal pressure existing in the space.
In the case of some conventional relative pressure measuring transmitters, the reference pressure supply runs from the relative pressure sensor into an internal space in the housing and ends there. The housing then has a bore leading to the outside, via which the reference pressure supply is connected to the environment of the relative pressure measuring transmitter.
The bore is routinely connected with a filter, which is supposed to prevent intrusion of moisture.
Depending on temperature of the environment, however, there is always a certain fraction of moisture contained in the medium surrounding the relative pressure measuring transmitter, be it air or a gas. Usual commercial filters are not able to completely filter this moisture out. Through an exchange of the medium, which surrounds the relative pressure measuring transmitter, moisture can, therefore, pass through the filter, into the interior space. The larger the internal space of the housing, the greater is the amount of medium exchanged in such case, and, concomitantly, the greater is the intruding moisture.
If the temperature in the environment is higher than the temperature inside the housing, then the dew point can be subceeded, or fallen beneath, inside the housing, this leading to the formation of condensate, which deposits inside the housing. The condensate accumulates in the housing.
Electronic circuits possibly present in the housing, and the relative pressure sensor, are, as a rule, very sensitive to moisture.